Process for the separation of lactic acid from water and nitric acid by rapid preevaporation and distillation

ABSTRACT

Lactic acid is recovered from aqueous mixtures containing it together with nitric acid, e.g. obtained by hydrolysis of the oxidation product of propylene with nitric acid, by passing the mixture into a rapid evaporator operating at a pressure less than 200 mm. mercury with a residence time less than 5 minutes to give a vaporized phase containing at least 60 percent by weight of the water and nitric acid and less than 40 percent by weight of the lactic acid, passing this vaporized phase together with nonvaporized material directly to a distillation column operating under reduced pressure and recovering the lactic acid as bottom product.

United States Patent [191 Chaintron 1March 20, 1973 1 PROCESS FOR THESEPARATION OF 2,543,001 2/1951 Dean ..202/1s3 LACTIC ACID FROM WATER ANDgitlmgre 7 s ra Ol' ggg gggggggggg Dam 2,334,524 11/1943 WenkerDISTILLATION 2,350,370 6/1944 Schopmeyer et al ..260/535 [75] Inventor:Gerard Chaimron, Lyon France 2,847,464 8/1958 Robertson et al. ..260/533[73] Assignee: Rhone-Poulenc S.A., Paris, France Primary Examiner-WilburL. Bascomb, Jr. [22] Filed, Nov 23 1970 A!torney--Stevens, Davis, Miller& Mosher [21] Appl. No.: 91,801 [57] ABSTRACT Lactic acid is recoveredfrom aqueous mixtures con- [30] Foreign Application Priority Datataining it together with nitric acid, e.g. obtained by Nov 24 1969France 6940393 hydrolysis of the oxidation product of propylene withnitric acid y p g the mixture into a rapid [52] U S C] 203/77 203/15203/80 evaporator operating at a pressure less than 200 mm. 205/15326O/535 mercury with a residence time less than 5 minutes to [51] Int ClC07c 59/08 give a vaporized phase containing at least 60 percent 58]Fieid 88 91 by weight of the water and nitric acid and less than 40 6 5i 5 percent by weight of the lactic acid, passing this vaporized phasetogether with non-vaporized material directly to a distillation columnoperating under [56] References Cited reduced pressure and recoveringthe lactic acid as bot- UNITED STATES PATENTS tom product- I,S94,8438/1926 Lawrie ..260/535 R 8 Claims, 2 Drawing Figures Mir/c 40/0 0/70Water PROCESS FOR THE SEPARATION OF LACTIC ACID FROM WATER AND NITRICACID BY RAPID PREEVAPORATION AND DISTILLATION The present inventionrelates to a process for recovering lactic acid from a mixture whichessentially comprises nitric acid, lactic acid and water.

French Patent Specification No. 1,465,640 and its Additions, Nos. 90,556and 92,1 16 and French Patent Specification No. 1,500,619, describe thepreparation of lactic acid by oxidation of propylene with nitric acidand/or nitrogen peroxide, optionally in the presence of oxygen, to givean intermediate product consisting principally of a-nitratopropionicacid, which is thereafter subjected to hydrolysis with water, whereappropriate in the presence of an acid such as nitric acid. During thereaction, nitric acid is liberated and at the end of the treatment anaqueous solution of lactic acid is obtained which contains nitric acidand possibly other reactants or reaction byproducts in minor amounts.

In order to prepare pure lactic acid, it is necessary to subject thismixture, in solution, to various separation processes.

According to French Patent Specification No. 1,500,619 the nitric acidcan be separated off either by means of liquid anion exchangers or byazeotropic distillation with the aid of acetic acid. These twoseparation processes are unsatisfactory in that they involve the use ofa third substance to effect the separation and also processes for thepurification or regeneration of this third substance.

A process has now been discovered in accordance with the presentinvention which permits the separation of lactic acid from aqueoussolutions of lactic and acetic acids without the involvement of thirdsubstances.

The present invention provides a process for the recovery of lactic acidfrom a mixture comprising lactic acid, nitric acid and water wherein themixture is passed into a rapid evaporator operating under reducedpressure, the rate of flow of the mixture into the evaporator and theheating of the evaporator being such that the mixture has a residencetime in the evaporator of less than minutes, preferably less than 1minute, and there is obtained a vapor fraction containing an amount ofnitric acid and water which is at least 60 percent, preferably at least80 percent by weight of the nitric acid and water originally present inthe mixture and less than 40 percent and preferably less than percent byweight of the lactic acid originally present in the mixture andthereafter, the vaporized fraction and the non-vaporized fraction arepassed directly to a distillation column operating under reducedpressure and distilled so that nitric acid and water issue from the headof the column and lactic acid, which may be partially or completely inthe form of lactyllactic acid, issues at the base of the column, thepressure reduction applied to the evaporator and column being such thatthe pressure in the evaporator is less than 200 mm. and preferably lessthan 100 mm. mercury.

Lactyllactic acid may be obtained in the present process in accordancewith the following equation:

Since this equilibrium is displaced in one direction or the otherdepending on the conditions and especially depending on the temperatureand the amount of water present (in the case of an aqueous solution oflactiri acid), it follows that generally lactic acid, even purified andsold commercially practically always contains varying amounts oflactyllactic acid [R. Eder. et al l-lelv. ChemJActa 9 355-64 1926)].

The mixture of nitric acid, lactic acid and water to which the processof the invention is applied may be one obtained by hydrolysis, inaccordance with the known processes, of the products of the oxidation ofpropylene by N 0 and/or HNO such as anitratopropionic acid. They canthus still contain minor amounts of reactants, of hydrolysis productsand/or of oxidation products, of a known or unknown nature.

Various apparatus can be used to carry out the process of the inventionbut will generally consist of a combination of at least one device whichallows a rapid, preferably instantaneous, evaporation, and adistillation column.

A more detailed description of the invention will now be given withreference to FIGS. 1 and 2 of the attached drawings which schematicallyillustrate, by way of example and without a specific scale, embodimentsof the apparatus which may be used.

FIG. 1 illustrates a so-called instantaneous" evaporator combined with adistillation column, and

FIG. 2 illustrates a different embodiment which employs twoinstantaneous evaporators.

In FIG. 1 instantaneous evaporator l, which can be of any known type;essentially comprising a tube 2 surrounded by a heating jacket 3, andhaving aqueous acid entry port 50, is arranged along an inclined orvertical axis, and opens into distillation column 20, at the middle zonethereof 4. Column 20 may be of conventional type provided, on theinside, with a device which allows intimate contact of the rising vaporswith the descending liquids; this column can thus be of the plate typeor of the packed type. The head of column 20 is provided with anoverhead products line 21 leading to condenser 22, the output from whichis partially recycled to column 20 through line 23 as reflux liquid andpartially removed from the system through outlet 51. The bottom of thecolumn 20 is provided with lactic acid outlet 52 and a boiler 25 withassociated lines 24 and 26 leading from and to column 20.

Before carrying out a separation according to the invention, theapparatus is evacuated to a reduced pressure of less than 200 mm ofmercury, preferably less than mm of mercury. During operation, thepressure is essentially the same throughout the apparatus, but smalldifferences can nevertheless exist between the instantaneous evaporatorl and the column 20 because of pressure losses.

Into this apparatus of FIG. 1, subjected to reduced pressure, themixture of nitric acid, lactic acid and water is introduced at 50; thismixture passes into the tube 2 and is heated by means of a heating fluidwhich flows through the jacket 3, so as to vaporize an amount of waterand nitric acid which is at least 60 vapor, by weight of that originallypresent and less than 40 percent of the lactic acid during the residencetime which is less than 5 minutes. A mixture of vapors and liquid isobtained, which at 4 passes into the middle part of the column: thevapor phase of this mixture essentially consistsof water and nitric acidand usually contains a little lactic acid; the liquid phase of themixture essentially consists of lactic acid and/or lactyllactic acid,but usually contains a little water and nitric acid; these two phases,namely the liquid and the vapor separate in the column, the formerpasses to the head and the latter to the base of column 20.

At the head of column 20, the vapors reach condenser 22 through line 21.The condensate obtained is partially recycled into column 20 throughline 23 and partially removed from the system through outlet 51.

At the base of column 20, a branch circuit consisting of lines 24 and 26allows boiler 25 to supply to the column the heat required for thedistillation; boiler 25 can operate by simple heating; it can also causetotal or partial vaporization of the liquid flowing through it. In thiscase, the boiler can consist of an instantaneous evaporator, which makesit possible, by reducing the residence time of the products in theapparatus, to obtain a lactic acid which is principally in the lactic,and not in the lactyllactic, form.

The lactic acid, possibly in the lactyllactic form, is withdrawn at 52.7

FIG. 2 illustrates apparatus comprising a two-stage rapid evaporationsystem in combination with a distillation column.

The two-stage rapid evaporation system principally comprises twoinstantaneous evaporators 1 and 10, and a separator 30, preferably ofthe cyclone type is provided. The output from evaporator 1 passes toseparator 30 and the more volatile products separated pass through line31 to the column 40 at 4. The less volatile products leave separator 30through line 32 which feeds evaporator 10 and the output from evaporator10 enters column 40 at 5. Instantaneous evaporators l and 10 may be ofany known type, for example of the type described in relation to FIG. 1and for reasons which have again been explained in relation to FIG. 1,these evaporators are preferably arranged along an inclined or verticalaxis, with ascending circulation of the fluids flowing through them.

The column 40 only differs from column in FIG. 1 in that it has two feedtubes 4 and 5 in the middle zone in place of a single tube. Tube 4 isarranged closer to the top of column 40 than tube 5, because it feedsthe column with mixtures which are richer in water and nitric acid thanthose supplied by tube 5.

Column 40 like column 20 is provided with overhead products line 21,condenser 22, reflux line 23 and outlet 5] at the head of the columnandlactic acid outlet 52, boiler and associated lines 24 and 26 at thebottom of the column.

After evacuating the apparatus of F lG. 2, to provide reduced pressureas described above in relation to the apparatus of FIG. 1, the mixtureof nitric acid, lactic acid and water is introduced at 50 intoinstantaneous evaporator 1. On issuing from this evaporator, the mixtureof liquids and vapors passes tangentially into cyclone 30. The vaporphase, consisting principally-of water and of nitric acid, escapesthrough the line 31 and passes into distillation column 40 at 4. Theliquid phase, which principally comprises lactic acid, possibly in thelactyllactic form, and usually contains a little water and nitric acid,flows from cyclone 30 through line 32 and to evaporator 10 where it issubjected to a fresh rapid evaporation; the two rapid evaporationscarried out in evaporators l and 10 being such that, at the end of thetwo evaporations, at least 60 percent of the water and nitric acidinitially present in the mixture, and less than 40 percent of the lacticacid in the mixture, have been vaporized. The residence time of themixture in the part of the apparatus which precedes the distillationcolumn is in all cases less than 5 minutes. From the outlet ofevaporator 10, a mixture of vapors is obtained, usually with a littleliquid, which enters the distillation column 40 at 5.

As in the apparatus shown in FIG. 1, after distillation, the water andnitric acid are withdrawn from the system at 51 whilst the lactic acid,which can be wholly or partly in the lactyllactic form, is withdrawn at52.

Apparatus other than those described above can of course be used withoutgoing outside the framework of the present invention.

For example, the rapid evaporation system combined with the distillationcolumn can comprise more than two instantaneous evaporator stages.

As instantaneous evaporator, a rotating evaporator,

bundle-type evaporator, plate-type evaporator, or

spiral cube-type evaporator, is preferably used.

Steam, or heating fluids in the liquid or vapor phase, can be used asthe heating fluid; where appropriate, the wall of the evaporator can beheated directly by electrical resistances or by any other suitablemeans.

In the case of an apparatus comprising several evaporator stages, theseparator or separators arranged between the stages can be of variousknown types, such as filters, mechanical separators, and the like.

The reflux device at the head of the distillation column, and the boilerat the base of the column, are not essential, but it is neverthelesspreferred to use them to improve the efliciency of the column.

The process according to the invention is of particular value because itcan easily be operated continuously. It yields a lactic acid which onlycontains amounts of nitric acid which are generally negligible;depending on the particular conditions employed, and especially on theaverage residence times of the liquids at the base of the distillationcolumn, the lactic acid obtained can be partially or wholly in the formof lactyllactic acid.

The following Examples are given to illustrate the invention.

EXAMPLE I The tube 2, which has an internal diameter of 30,

mm. and is 100 cm. long, is heated by means of a steam circulationsystem which provides 3200 kcal/hour (temperature 100C).

The average residence time of the mixture in the instantaneousevaporator is about 0.02 seconds.

The vapors issuing from this evaporator contain percent of the water andnitric acid initially present in the mixture, and 5 percent of thelactic acid originally present.

The column 20 is a column of 16 cm. diameter and 3 m. height, comprisingseven bubblecap plates, the feed tube at 4 being located between thesecond and third plate, counting from the bottom. The reflux ratio, thatis to say the ratio by weight of condensate recycled at 23 to the weightof liquid issuing from the column at 51, is 0.2.

At 51, a mixture of water and nitric acid containing about 0.1 percentof lactic acid is withdrawn at the rate of 8.5 kg./hour. At 52 lacticacid, essentially in the lactyllactic form, containing about 0.1 percentof nitric acid and of water, is withdrawn at the rate of 1.5 kg./hour.

EXAMPLE 2 A mixture of nitric acid/lactic acid/water in respectiveratios by weight of 40/ 1 5/45, is introduced at 50, at the rate ofkg./hour and at a temperature of C., into an apparatus of the type shownin FIG. 2. The absolute pressure in the apparatus is 40 mm. of mercury.

The two evaporators 1 and 10 and the column 40 are similar to thecorresponding elements described in Example l. The inlet 4 islocated'between the second and the third plate of the column, countingfrom the bottom; the inlet 5 is located between the first and the secondplate. The cyclone has a diameter of 16 cm. and a height of 20 cm.

The various fluids, whether liquids or vapors, are circulated asdescribed above with reference to FIG. 2, and in a similar mannermutatis mutandis to that described in Example 1. The reflux ratio is0.2.

The evaporator l, heated with steam at 100C., provides 2850 kcal/hour;the vapor issuing from this evaporator contains 80 percent of thewater-nitric acid combination originally present in the mixture and 2percent of the lactic acid.

The evaporator 10, heated with steam at 130C provides 700 kcal/hour; thevapor issuing from this evaporator contains 80 percent of thewater-nitric acid combination and 13 percent of the lactic acid presentin the liquid fed to evaporator 10. I

The average residence time of the products between inlet 50 intoevaporator 1 and inlet 4 into column 40 is about 0.03 seconds.

The average residence time of the products between inlet 50 and inlet 5into column 40 is about 10 seconds.

At outlet 51 of the apparatus, a water-nitric acid mix-' ture containingabout 0.1 percent of lactic acid is collected at the rate of 8.5kg./hour.

At outlet 52 of the apparatus, lactic acid, essentially in thelactyllactic form, containing about 0.1 percent of a mixture of waterand nitric acid, is collected at the rate of 1.5 kg./hour.

lclaim:

l. A process for the recovery of lactic acid from a mixture comprisinglactic acid, nitric acid and water wherein the mixture is passed into arapid evaporator operating under reduced pressure, the rate of flow ofthe mixture into the evaporator and the heating of the evaporator beingsuch that the mixture has a residence time in the evaporator of lessthan 5 minutes, and there is obtained a vapor fraction containing anamount of nitric acid and watenwhich is at least 60 percent by weight ofthe nitric acid and water originally present in the mixture and lessthan 40 percent by weight of the lactic acid originally present in themixture and thereafter, the vaporized fraction and the nonvaporizedfraction are passed without separation from one another directly to adistillation column operating under reduced pressure and distilled sothat nitric acid and water issue from the head of the column and lacticacid, which may be partially or completely in the form of lactyllacticacid, issues at the base of the column, the pressure reduction appliedto the evaporator and column being such that the pressure in theevaporator is less than 200 mm. mercury.

2. A process according to claim 1 wherein the residence time of themixture in the evaporator is less than 1 minute.

3. A process according to claim 1 wherein the pressure in the evaporatoris less than 100 mm. mercury.

4. A process according to claim 1 wherein the vaporized product containsat least percent by weight of the nitric acid and water in the mixtureand less than 20 percent by weight of the lactic acid in the mixture.

5. A process according to claim 1 wherein the mixture is obtained byhydrolysis of the product obtained by oxidizing propylene with nitricacid and/or nitrogen peroxide.

6. A process according to claim 1 wherein the evaporator is connected tothe middle zone of the column and is located along an inclined orvertical axis so that the fluids flowing through it circulate in anascending manner.

7. A process according to claim 1 wherein the mixture is a mixture ofnitric acid, lactic acid and water in a weight ratio of about 40:15:45,and is fed into a rapid evaporator operating at about 100C. at apressure of about 40 mm. mercury with a residence time in the evaporatorof about 0.02 seconds to give a vaporized fraction containing aboutpercent by weight of the water and nitric acid and 5 percent by weightof the lactic acid present in the original mixture.

8. A process according to claim 1 wherein the mixture is a mixture ofnitric acid, lactic acid and water in a weight ratio of about 40:15:45and is fed into a rapid evaporator operating at about C. at a pressureof about 40 mm mercury to give a vaporized fraction containing about 80percent by weight of the nitric acid and water and about 2 percent byweight of the lactic acid present in the original mixture which ispassed to the distillation column and a non-vaporized fraction which ispassed to a second rapid evaporator operating at about C. at a pressureof about 40 mm mercury to give a second vaporized fraction and anonvaporized fraction, both of which are fed to the distillation column.

2. A process according to claim 1 wherein the residence time of themixture in the evaporator is less than 1 minute.
 3. A process accordingto claim 1 wherein the pressure in the evaporator is less than 100 mm.mercury.
 4. A process according to claim 1 wherein the vaporized productcontains at least 80 percent by weight of the nitric acid and water inthe mixture and less than 20 percent by weight of the lactic acid in themixture.
 5. A process according to claim 1 wherein the mixture isobtained by hydrolysis of the product obtained by oxidizing propylenewith nitric acid and/or nitrogen peroxide.
 6. A process according toclaim 1 wherein the evaporator is connected to the middle zone of thecolumn and is located along an inclined or vertical axis so that thefluids flowing through it circulate in an ascending manner.
 7. A processaccording to claim 1 wherein the mixture is a mixture of nitric acid,lactic acid and water in a weight ratio of about 40:15:45, and is fedinto a rapid evaporator operating at about 100*C. at a pressure of about40 mm. mercury with a residence time in the evaporator of about 0.02seconds to give a vaporized fraction containing about 90 percent byweight of the water aNd nitric acid and 5 percent by weight of thelactic acid present in the original mixture.
 8. A process according toclaim 1 wherein the mixture is a mixture of nitric acid, lactic acid andwater in a weight ratio of about 40:15:45 and is fed into a rapidevaporator operating at about 100*C. at a pressure of about 40 mmmercury to give a vaporized fraction containing about 80 percent byweight of the nitric acid and water and about 2 percent by weight of thelactic acid present in the original mixture which is passed to thedistillation column and a non-vaporized fraction which is passed to asecond rapid evaporator operating at about 130*C. at a pressure of about40 mm mercury to give a second vaporized fraction and a non-vaporizedfraction, both of which are fed to the distillation column.